As spindles and bearings get smaller, requirements for high precision performance increase and the need for the sealing of the annulus around a shaft arises, it becomes increasingly difficult to utilize conventional design in the field of bearings and seals.
Accordingly, efforts have been made to use magnetic fluid seals to contain or prevent the magnetic fluid from migrating outside the region of the bearing and potentially contaminate the exterior region, which may contain such devices as magnetic data storage disks. With the use of relatively large size shafts, ball bearings and fluid bearings have been preferred to reduce friction between the hub and the shaft. With ball bearings, there is relatively low friction; however, ball bearings are lubricated with oils or greases. The magnetic seal may be a separate element of the assembly, merely preventing the lubricants of the ball bearing and the air contained within that chamber from passing coaxially to the shaft.
As shaft diameters are reduced along with the size of the devices within which they are contained, the practicality of miniature ball bearings becomes an issue. In some cases, the shaft diameters are reduced below two millimeters and, accordingly, the normal tolerances for the miniature ball bearings become so great in proportion to the component sizes that accuracy of positioning of the rotating element is degraded beyond acceptable limits
Efforts have been made to utilize fluid bearings in place of the ball bearings with varying degrees of success. The stability of the revolving hub requires that the bearings be spaced apart from each other along the axis of the shaft to the greatest possible extent The separation of the bearings requires that either each individual bearing be designed for individual containment, thereby typically requiring an end plugging scheme to contain the fluid in the bearing; or alternatively, the use of magnetic seals to confine the fluid in the bearing cavity. Thus, in the past, the use of magnetic seals for each bearing has embodied two magnetic seals for each bearing or, alternatively, the inclusions of two or more bearings within a common fluid cavity sealed by two seals, one at each end of the fluid cavity. The inclusion of more than one bearing within the single cavity dictates that the cavity extend over substantial lengths to accommodate the multiple bearings. To fill this cavity with magnetic fluid becomes cost significant in view of the exceedingly expensive cost of the magnetic fluid used as a lubricant in the bearings and as a sealing fluid in the seals.
Any compromise in the length of the chamber in order to reduce the fluid capacity requires that the bearings be displaced closer together, thus degrading the stability of the rotating hub surrounding the shaft. Degraded hub stability directly correlates to degraded or failed operability of the disks attached to the hub.
Inasmuch as the hub, at least in the preferred embodiment, supports magnetic storage data disks which are rotated at high speed and these disks are radial flanges mounted on the exterior of the hub, the stability of the rotating hub is exceedingly important to prevent the flanges' fluctuation relative to the position of the read/write heads associated with the disks. Fluctuation of the disk surfaces during rotation can cause collisions between the disks and the read/write heads mounted in exceedingly close proximity thereto, thereby damaging the disks and/or the heads with resulting loss of stored data.
With the bearings displaced from each other as far as the disk drive assembly design permits, the stability of the hub can be maximized and undesired displacement of the read/write point on a disk minimized by the use of close tolerances and a fluid bearing.
Another consideration which is key to the operation and reliable recording of data on the disks is the control of magnetic flux. The disks are magnetic material coated for receiving electromagnetic signals and storing those electromagnetic patterns; it is essential that the stray magnetic flux not be permitted to influence the magnetic coating of any of the magnetic disks carried on the revolving hub.
With this in mind, it can be clearly seen that the use of large or very strong magnets in the magnetic circuit is undesirable inasmuch as these strong magnets may propagate stray flux at substantial distances within the housing. Inasmuch as the dimension of the shaft diameter is two millimeters and typically smaller, it can be seen that the magnet placed in a sealing circuit very easily could propagate stray flux lines passing a substantial distance away and inadvertently affect the magnetic material coating of the magnetic storage disks.
Several approaches to sealing and capturing magnetic fluid in the region of a fluid bearing are known. One approach utilizes a single magnetic seal and a physical barrier to contain the magnetic fluid within the bearing.
Examples wherein a magnetic seal is used to contain magnetic fluid within a cavity where the other end of the cavity is a physical barrier to the migration or loss of the magnetic fluid are U.S. Pat. Nos. 4,526,484 to Stahl et al.; 4,734,606 to Hajec, and 4,938,611 to Nii et al. Stahl et al. utilizes a magnetic seal to contain the magnetic fluid within a cylindrical opening formed into a block where the cylindrical opening terminates within the block and there is no second opening thereto.
Hajec utilizes a screw-threaded plug member which is inserted into the main housing, which acts as a thrust bearing surface.
The Nii et al. reference shows a closed well formed by the bearing housing elements which contains the magnetic fluid in addition to the containment of the magnetic seal structure. The magnetic seal structure only encircles the shaft at one location along its axis point.
An effort has been made to seal fluid bearings utilizing magnetic fluid on both ends of the bearing by the utilization of a single magnet and two pole pieces, as illustrated in FIG. 2 of U.S. Pat. No. 4,598,914. This figure is labeled as prior art to the patent in which it appears and its origin is unknown.
FIG. 2 of U.S. Pat. No. 4,598,914 illustrates a seal arrangement forming a magnetic circuit around and containing the bearing surfaces of a fluid bearing. The arrangement disclosed and described utilizes a single magnet and two annularly shaped pole pieces. The single magnet is a hollow tubular magnet surrounding the bearing at some substantial distance from the bearing surface. The magnet's interior cylindrical surface supports a non-magnetic bearing material likewise formed in a hollow cylindrical shape surrounding the bearing with the inner surface of the bearing material in close proximity to the exterior surface of the bearing on the shaft.
The washer shaped or annular pole pieces act to focus the magnetic flux from the ends of the magnet cylinder into close proximity with and into the shaft creating a high flux density in the gap between the pole pieces and the shaft. In this regard, the magnetic fluid is trapped in the two gaps between the pole pieces and the shaft and in the gap between the bearing surface on the shaft and the bearing surface of the non-magnetic bearing material carried by the magnet. In order to insure that an effective flux density will be present between the pole pieces and the shaft, a strong and relatively large magnet is required due to its displacement from the sealing gaps.
With such a strong and relatively large magnet, the stray flux, which is inherent with a magnet, will tend to branch outside the bounds of the magnet and the pole pieces and possibly to adversely effect any magnetic disks which may be attached to the housing.
A second problem is encountered when utilizing the arrangement described immediately above because FIG. 2 of U.S. Pat. No. 4,598,914 illustrates a housing, a magnet and a non-magnetic bearing material assembled together with the magnet sandwiched between the non-magnetic bearing material and the housing. In the typical environment requiring fluid bearings, such as in small confined areas and those areas requiring extreme precision, the parallelism of the interior and exterior cylindrical surfaces of the magnet and the non-magnetic bearing material and the interior cylindrical surface of the frame are all extremely critical. The introduction of an additional member and any variables associated with it, over and above that which is absolutely essential, is highly undesirable. The undesirability of that arrangement lies in tolerance build-up and non-parallelism of any of the surfaces discussed above which will then potentially result in bearing failure, due to the fact that the gap between the inner and outer bearing surfaces is varied in an axial direction thus resulting in inadequate lubrication and hydrostatic pressure at one end or the other of the bearing, potentially resulting in early bearing failure.
Other attempts to solve this problem include U.S. Pat. Nos. 4,630,943 and 4,673,997. Both of these patents illustrate bearings and seals displaced from each other axially along a shaft with a complete magnetic seal and magnetic circuit at each end of the cavity. Such an arrangement requires filling of the bearing cavity over its entire length and the use of relatively large quantities of very expensive magnetic fluid. Each magnetic seal at each end of the cavity is a complete magnetic circuit in and of itself and independent of the other seal at the other end of the cavity.